Slow sand filters are known to be effective in removing bacteria, parasites, Giardia cysts, Cryptosporidium oocysts and viruses. While bacteria, parasites and viruses may be killed or rendered inactive with disinfection, dormant cysts and oocysts are very resistant to disinfection unless disinfectant is administered very intensively. Slow sand filters may be used to supply clean water directly for consumption or may be used to supply water for further treatment, such as by reverse osmosis and ultra-filtration, both of which processes require quite clean water to begin with.
In the prior art, slow sand filters are known that require a continuous water supply, and that are intended for use with continuously flowing water. Such a prior art slow sand filter is described in U.S. Pat. No. 5,032,261, to Pyper. The slow sand filter includes a container partially filled with sand. The slow sand filter is provided with a water inlet and water outlet. Water containing oxygen continuously flows through the sand at a rate of about or slightly over 0.08 m/hour, with the sand providing a filter effect for the flowing water. In addition, when the sand is immersed in the oxygen containing flowing water for several days, a beneficial aerobic initially barely visible biolayer forms at the top of the sand. This layer is referred to as a schmutzdeuke, which is formed from various organisms such as algae, plankton, diatoms, protozoa and bacteria. The schmutzdeuke entraps and partially digests and breaks down organic matter contained in the raw water continuously passing through the filter. The organic matter accumulates on the schmutzdeuke and simple inorganic salts are formed. At the same time, inert suspended particles in the raw water are removed mechanically by the sand.
Such slow sand filters are not, however, effective where the water supply is intermittent, such as at country cottages. In such cases, the water in prior art slow sand filters becomes stagnant (loses oxygen) and the schmutzdeuke dies due to lack of oxygen. This shortcoming limits the utility of prior art slow sand filters.
U.S. Pat. No. 4,765,892, Aug. 23, 1988, Hulbert et al., discloses a slow sand filtering system in which an uncovered filter tank has therein a layer of sand, and an effective filter-enhancing amount of zeolite. The zeolite is topped by a growth of schmutzdeuke, with the schmutzdeuke comprising a layer of living organisms selected from the group consisting of algae, plankton, diatoms, protozoa, rotifers and bacteria.
Thus, in Visscher et al., "Slow Sand Filtration for Community Water Supply", Technical Paper No. 24, International Reference Centre for Community Water Supply and Sanitation, The Hague, The Netherlands, 1987, at page 31, it is stated that intermittent operation of slow sand filters should not be permitted because "it has been shown conclusively that an unacceptable breakthrough of bacteriological pollutants occurs four to five hours after the filters recommence operation". Likewise, Huisman, "Slow Sand Filtration", World Health Organization, Geneva, 1974, at page 32, teaches that slow sand filters should be operated at as constant a filtration rate as possible. Thus the understanding in the art was that slow sand filters should not be operated intermittently. By intermittently, it is not meant that the slow sand filter be occasionally taken out of operation for maintenance, such as is described in U.S. Pat. No. 4,765,892 of Hulbert, but that the slow sand filter remains in operation, with the schmutzdeuke alive, during the period when there is no flow of water through the filter. By contrast with the present invention, the filter of Hulbert is cleaned during maintenance, so that the schmutzdeuke does not remain alive until it is operated again.
For a slow sand filter of the type described by Hulbert, with depth of water above the filter material being in the range of 60 cm to 2.44 m, the schmutzdeuke will begin to die immediately as the water flow is shut off during periods of intermittent operation and will die overnight due to lack of oxygen. This can be shown from theoretical calculations of the oxygen demand of the schmutzdeuke.